You're pretty much 'spot on' which my own feeling about the [apparently] young internet community. Though, I've seen older and relatively matured people (over 50 years old) type like this too. I'm wondering if it in some way correlates with an Internet Maturity or some other factor rather than actual age.
However, every thought I have on the subject brings forth two or three more questions and other points to ponder.
I tend to feel it's also an issue with lack of respect (self and of others) and discipline. People today are taught, especially by our media, to be aggressive, violent, and even disrespectful to peers and especially older people.
When I read posts or emails and I see the type of shorthand you're describing, I often get a feeling the writer is unintelligent, lazy, and or undisciplined. Even if this is not the truth about the person typing the message, that's the general impression I get.
My ponderings, again, bring me back to this correlating to some time of 'internet maturity'. For someone like yourself (and myself), we're confident enough to maintain our ideals and social screeners/filters while communicating online. Someone very new to the internet and who does not necessarily have the desire or capacity to learn the concepts tend to hang out in online chat room communities. In those communities of like-minded and often immature or inexperienced computer users, behavior like what we’re describing appears to often flourish and take root.
I think the subject is very interesting and I’d love to read some of the sociological studies made on behavior online.